1. Field of the Invention
The present invention relates to a vehicle power supply device in which a capacitor is disposed parallel to a secondary battery.
2. Description of the Related Art
It has been proposed in the related art that a capacitor such as an electrolytic capacitor or an electric double layer capacitor be disposed parallel to a secondary battery such as a lead battery in a power supply device for various types of vehicles such as a gasoline vehicle, a diesel vehicle, a hybrid vehicle, and an electric automobile. When this type of power supply device is installed in an electric automobile or the like, for example, energy generated during vehicle deceleration can be regenerated efficiently as electric power, enabling an extension of the range that can be traveled in a single charging cycle. Meanwhile, in a gasoline vehicle or a diesel vehicle, for example, an improvement in fuel efficiency can be achieved through a comparatively simple constitution by varying a generated voltage of an alternator to control charge and discharge to and from the capacitor, and as a result, effects such as an improvement in the starting property of a so-called idling stop function can be achieved without the need for size increases and the like in the secondary battery.
In this type of power supply device, a relay (main relay) is typically provided to release the capacitor electrically from the secondary battery in cases such as when an ignition switch (IG SW) is switched OFF or an abnormality occurs in the capacitor. To prevent deterioration of the capacitor caused by the voltage applied thereto, discharge control is typically performed on the capacitor when the main relay is opened after the IG SW is switched OFF. Further, to prevent the occurrence of a large inrush current while the main relay is ON due to a potential difference between the discharged capacitor and the secondary battery or the like, a precharge relay is provided alongside the main relay to connect the capacitor to the secondary battery via a current limiting resistor. By switching the precharge relay ON in advance of the main relay to precharge the capacitor, the potential difference between the capacitor and the secondary battery is alleviated.
As an example of a technique for reducing the frequency of precharging performed in this manner, Japanese Unexamined Patent Application Publication H9-149509 discloses a technique in which discharge of the capacitor is prohibited until a key is removed from a key cylinder while the IG SW is OFF.
Incidentally, demands have been made for an increase in the capacity of the capacitor in this type of power supply device from the viewpoint of improving the energy regeneration efficiency and so on. However, increases in the capacity of the capacitor lead to increases in the amount of power that is lost by discharging the capacitor when the IG SW is switched OFF and the amount of time required for precharging before the main relay is switched ON.
Hence, particularly when the vehicle is driven for a short time and then left stationary with the key switched OFF repeatedly, the frequency with which opportunities for energy regeneration and so on are lost increases during precharging, and moreover, the amount of power lost due to discharge of the capacitor increases, and as a result, energy efficiency may deteriorate.